1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, more particularly to a method of depositing tungsten in holes in a semiconductor device.
2. Description of the Related Art
Tungsten (W) is a conductive metal that is commonly used to fill the contact holes or via holes that pass through dielectric films to interconnect wiring traces in different wiring layers in semiconductor devices.
The known methods of depositing tungsten include chemical vapor deposition (CVD), sputtering, and atomic layer epitaxy (ALE). The most widely used method is tungsten CVD, a description of which is given by Miyata in Japanese Patent Application Publication No. H10-209280. This method employs, for example, the reaction between tungsten hexafluoride gas (WF6) and hydrogen gas (H2) given by the following formula:WF6+3H2→W+6HF
When this reaction is used to deposit tungsten in a hole under conditions such that the deposition rate is equal to the supply rate (such conditions occur in region A in the graph in FIG. 1), the reaction tends to be completed near the surface of the dielectric film. The tungsten atoms therefore have a high probability of being deposited on the surface of the dielectric layer or the top inner rim of the hole, and a low probability of penetrating into the deeper parts of the hole. The result can be that insufficient tungsten reaches the interior of the hole and a void is left, as seen in FIG. 2, which shows the dielectric film 14, an adhesion layer 16 of titanium nitride (TiN), the hole 18, and the void 20.
Under conditions such that the deposition rate is equal to the reaction rate (such conditions occur in region B in FIG. 1), WF6 gas is supplied in sufficient quantity that tungsten is deposited at a uniform rate on both the surface of the dielectric film and the interior of the hole, as shown in FIG. 3. Under these conditions, the hole can be filled with tungsten without forming a void.
Tungsten CVD is therefore usually carried out under region B conditions, but WF6 gas also has the unwanted effect of etching the deposited layer of tungsten, the substrate layer on which the tungsten is deposited, and the dielectric film in which the hole is formed. If more WF6 gas is supplied than necessary, the tungsten layer W and the TiN adhesion layer 16 in FIG. 3 may be etched to such an extent that detachment occurs. In addition, WF6 gas may diffuse through these layers and etch the dielectric film 14 or the layer below the dielectric film 14, creating voids there. The flow rate of the WF6 gas must be controlled to prevent these effects.
Ideally, it should always be possible to fill holes with tungsten as in FIG. 3, without creating voids, by performing tungsten CVD under region B conditions, but as the dimensions of wiring traces and the diameter of via holes become increasingly small, or the holes become increasingly deep, there is an increasing tendency for voids to form as in FIG. 2 despite the use of region B conditions. The reason is thought to be a decreasing probability of WF6 penetrating into the hole, especially to the bottom of the hole, so that in the interior of the hole, and especially at the bottom, tungsten is deposited under region A conditions, leading to a difference in the deposition rates at the top and bottom of the hole.
For tungsten CVD to proceed under region B conditions from the top to the bottom of the hole, it becomes necessary to increase the WF6 flow rate, but this leads to unwanted etching and the other problems noted above.
A proposed method of avoiding these problems is to deposit tungsten by atomic layer epitaxy, which has excellent coverage properties, but this method also has an extremely slow deposition rate, and is unsuitable for high-volume manufacturing.